Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a coating of a film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used in microlithographic processes. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the photoresist composition exposed to the radiation become less soluble to a developer solution while the unexposed areas of the photoresist coating remain relatively soluble in such a solution. Thus, treatment of an exposed negative-working photoresist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating, thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was formed.
After development of the photoresist film, the now partially unprotected substrate may be treated with a substrate-etchant solution, plasma gases, or have metal or metal composites deposited in the spaces of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a patterned substrate surface.
In the manufacture of patterned structures, such as wafer level packaging, electrochemical deposition of electrical interconnects has been used as the density of the interconnects increases. Gold bumps, copper posts and copper wires for redistribution in wafer level packaging require a resist mold that is later electroplated to form the final metal structures in advanced interconnect technologies. The photoresist layers are very thick compared to the photoresists used in the Integrated Circuit manufacturing of critical layers. Both feature size and photoresist thickness is typically in the range of 5 μm to 100 μm, so that high aspect ratios (resist thickness to line size) have to be patterned in the photoresist.
Devices manufactured for use as microelectromechanical machines also require very thick photoresist films in order to define the components of the machine.
It is the object of the present invention to provide a negative photoresist useful for imaging thick photoresist films, preferably as thick as 200 microns, which provide good lithographic properties, particularly photosensitivity, high aspect ratio, vertical sidewalls, improved adhesion on metal and silicon substrates, compatibility with electroplating solutions and process, reduced photoresist film cracking, ease and clean removal of the photoresist film after processing, and improved environmental stability. The applicants of the present invention have found that a photoresist comprising an alkali-soluble resin, where the polymer comprises at least one phenolic group derived from a hydroxystyrene monomer, at least one photoinitiator, and, at least one photopolymerizable monomer comprising 2 or more (meth)acrylate groups provides the desired lithographic properties when imaged, especially for thick films up to 200 microns.